1. Field of the Invention
The present invention relates to a gas-sensitive device with a characteristic gas-sensitive property which can prevent the reduction in the gas-sensitivity during its use and which is stable for a long period.
2. Background Information
Conventionally, various studies have been made on gas-sensitive substances such as oxide semiconductor, etc., which change their resistance in contact with various kinds of gases. In such gas-sensitive devices, catalysts are used for improving sensitivity and selectivity. For example, a certain gas-sensitive device is constructed to have a catalytic layer on its gas-sensitive substance, However, the catalyst may gradually diffuse into the gas-sensitive substance of the gas-sensitive device having this structure. Consequently, such a device may not have a stable gas-sensing property for a long period, which has induced various attempts for the improvement of the above drawbacks. One of the proposed solutions is illustrated in the Japanese Patent Application Laid-Open No. 59-99243 which employs a thin film containing Al.sub.2 O.sub.3, etc. as a carrier and a metal as a catalyst.
As is described in "the Background of the Invention" and "the Problems that the Invention is to solve" sections of the Japanese Patent Application Laid-Open No. 59-99243, a catalytic layer of a thick-film type is formed through the process of application and sintering of a material in paste. Therefore, the thus obtained layer does not show good reproductivity. There is also pointed out a problem that there may be observed some deviation in the gas-sensitive property of the obtained layer. A gas-sensitive device generally equipped with a heater detects gas while heating a gas-sensitive substance. When the film is thick as in this case, a temperature gradient may be induced in the catalytic layer whereby thermal stress may be induced, causing the occurrence of a crack, etc. in the catalytic layer. The above-described drawbacks have been solved by preparing the catalytic layer into thin film. Nevertheless, the catalytic layer still needs carrier such as Al.sub.2 O.sub.3, etc., so that essentially its structure must be complex. Furthermore, the device of the Japanese Patent Application Laid-Open No. 59-99243 has a sensitivity to 200 ppm CO gas of about 30, meaning that there has not yet been obtained a device with a high sensitivity.